All bit instructions (BSET, BCLR, BRSET, BRCLR) expect a zero-page operand (i.e., a variable mapped somewhere in page 0, RAM address 0 through 255).
Apparently, in your case, SCI1_S1 is located outside this range. So, you need to emulate the bit instructions. Example:
BCLR 7,XXX
is (practically) equivalent to
LDA XXX
AND #$7F
STA XXX
while a BRCLR 7,XXX,Address can be emulated with
LDA XXX
BIT #$80
BEQ Address
(Similarly for the rest, except that you need ORA for BSET and inverted mask, and BNE for BRSET.)
Hope this helps.